In cryogenic hydrogen storage systems, an accurate calculation method for the heat release from different ortho-para hydrogen catalytic conversions is important to determine the load of hydrogen storage systems. The objective of this study is to present a precise calculation method for the conversion heat of ortho-para hydrogen. By establishing a conversion model, the methods of using a smoothing spline curve to fit the experimental data and energy balance calculations are used to derive the released heat in adiabatic conversion multistage converters and the released heat during continuous conversion. The heat release properties of different conversion methods are analyzed in this study. Notably, the heat release amount of continuous conversion is the smallest, that of isothermal conversion is the largest, and the conversion heat of adiabatic conversion is in between, which is related to the number of conversion stages. In addition, a specific method and procedure are programmed to solve the implicit differential formula for continuous conversion. Finally, the calculation results of the different methods are consistent with a maximum deviation of only 0.22%, indicating that the calculations in this study are valid and accurate.